Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise electrode components, including an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reaction.
The electrocatalyst may be a metal black, an alloy, or a supported metal catalyst. The support for the electrocatalyst particles may be carbon particles. Preferably the carbon particles are in the form of a furnace black, although other particle forms may be used, such as an agglomerate, for example, an acetylene black. The electrode may also comprise other electrode components such as an electrically conductive substrate upon which the electrocatalyst layer is deposited. A typical substrate comprises a porous sheet material, such as carbon fiber paper or cloth. The electrode components may further comprise other electrically conductive particles such as fillers and binders. Some, or all, of the electrode components may be microporous.
For greater clarity, the term microporous is used in this disclosure to describe materials which have pores or crevices with an aperture size less than 1 micron. In this specification, an aperture is defined as the gap distance at any depth between opposing walls within pores. Accordingly, the term micropore is used to describe portions of pores or crevices with aperture sizes less than or equal to 1 micron.
A solid polymer fuel cell is a type of electrochemical fuel cell which employs a membrane electrode assembly ("MEA"). The MEA integrates a solid polymer electrolyte or ion-exchange membrane between the anode and the cathode. With such membranes, the water content within the membrane affects the performance of the fuel cell. The ion conductivity of the membrane generally increases as the water content or hydration of the membrane increases. For this reason, it is desirable for water to be present within solid polymer fuel cells. In some fuel cells, one or both of the reactant streams are humidified to prevent dehydration of the membrane.
Water is also produced within the fuel cell as the product of the cathode reaction. Oxygen is typically supplied to the cathode as pure oxygen or as a dilute oxygen stream such as air. Protons produced at the anode migrate through the ion conductive membrane and combine with oxygen at the cathode to produce water. Electrons are conducted from the anode to the cathode through an external circuit.
With direct methanol fuel cells, the fuel stream is typically aqueous methanol, with the larger portion of the fuel stream being water.
Since the anode and cathode are both adjacent to the hydrated membrane, the electrodes are consistently in contact with a water source. At the anode, humidified gaseous fuel streams or aqueous fuel streams are additional sources of water. At the cathode, reaction product water is an additional source of water. Accordingly, water typically collects or condenses within the pores of untreated porous electrode components.
The presence of water within the pores of electrode components is potentially harmful. However, if the temperature within the fuel cell falls below the freezing temperature of water for a sufficient time, the water may freeze. Since water expands when it freezes, when water freezes within the pores of fuel cell components the freeze-expansion may cause structural damage to porous fuel cell components. Such structural damage may result in deterioration of the electrode causing fuel cell performance degradation. If the water freeze-thaw cycle is repeated, the potential for freeze-expansion damage to porous fuel cell components increases. Porous electrode components are particularly at risk because water typically condenses within the pores.
It is known to apply surface coatings to porous electrodes for various reasons, such as for example, increasing the electrochemically active surface area of the electrocatalyst by applying an ionically conductive material to the electrode. A surprising result is that some of these coating materials when impregnated into the pores of the electrode components can improve the tolerance of porous electrodes to water freeze-thaw cycles. Such coating materials reduce the open pore volume which means that less water can collect within the pores. Coating materials deposited within the pores may also reduce the pore aperture size, making it more difficult for water to enter the pores. Some coatings may also be hydrophobic, such that the coatings actually repel water. Other coatings may have the effect of reducing the freezing temperature of water.
However, conventional methods of applying coatings do not coat all of the interior pore surfaces. Conventional methods may deposit a coating on the interior surfaces of some of the macropores which have aperture sizes larger than 1 micron. Conventional methods also impregnate some of the micropores. However, it has been found that the micropores typically comprise two distinct size ranges, generally referred to as primary micropores and secondary micropores. Primary micropores are smaller than secondary micropores. The primary micropores are not significantly impregnated by conventional methods, whereas conventional coating methods may impregnate a significant portion of the secondary micropores. The boundary aperture size between the primary and secondary micropores may depend upon the type of particles. For example, for carbon black particles, the boundary between primary and secondary micropores for furnace blacks is an aperture size of about 0.1 micron, whereas the boundary between primary and secondary micropores for acetylene blacks is an aperture size of about 0.04 microns. Conventional methods of coating electrodes are unable to significantly impregnate primary micropores.
Because the electrochemical reactions within an operating fuel cell are exothermic, with proper insulation, a conventional fuel cell may operate without experiencing internal freeze-thaw cycles in environments where the exterior of the fuel cell housing is exposed to temperatures significantly less than the freezing temperature of water. However, there is a problem for conventional fuel cells which may be shut down and stored in cold environments, such as fuel cells installed in vehicles or stationary power generators. For example, bus fleets do not typically have indoor garage facilities to accommodate all of the buses in a fleet; thus some buses may be parked in a yard where they are exposed to cold environments, including temperatures below the freezing temperature of water. Accordingly, there is a need to provide fuel cell electrodes with reduced performance degradation after exposure to temperatures below the freezing temperature of water.